Method of electrostatic powder gravure printing and apparatus therefor



WW B. REPF ETAL METHOD OF ELECTROSTATIC POWDER GRAVURE PRINTING AND APPARATUS THEREFOR 3 Sheets-Sheet 1 Filed June 5, 1964 WWE i Wfi R. B. REIF E'ML METHOD OF ELECTROSTATIC POWDER GRAVURE PRINTING AND APPARATUS THEREFOR 3 shee'ts -Sheet 2 Filed June 5, 1964 Fmuw 2 l k w V n Emu W, MW a. mu ETAL Bfififimfififi METHOD OF ELECTROSTATIC POWDER GHAVUHE PRINTING AND APPARATUS THEREFOR Filed June 5, 1964. 3 Sheets-"Sheet 5 United States Patent 3,296,965 METHOD OF ELECTROSTATIC POWDER GRA- VURE PRINTING AND APPARATUS THEREFOR Robert B. Reif, Grove City, Ohio, Norman S. Cassel, Ridgewood, N.J., and Daniel Smith, Riverdale, N.Y., assignors, by direct and mesne assignments, to Interchemical Corporation, New York, N.Y., a corporation of Ohio Filed June 3, 1964, Ser. No. 372,226 9 Claims. (Cl. 101-170) This invention relates to a process for gravure printing with the aid of electrostatic forces, using powder as the printing material, and to a printing apparatus for carrying out the process.

Gravure printing has the advantages of being effective on a wide assortment of substrates and of producing high quality prints. Powder has the advantage over conventional inks of eliminating organic solvents with their attendant odors and fire hazards. Powder also permits the use of insoluble resins as well as soluble ones. Employing electrical fields for printing eliminates the need for actual contact between the substrate and the printing cylinder. Printing can be accomplished even when the printing cylinder is as much as 0.1 inch from the surface to be printed on. It is also possible to do con-- tact printing electrostatically. Thus prints may be effectively made on quite irregular surfaces and also on materials where contact would injure the material or the printing cylinder. For example prints can be made on wet substrates. The powder gravure process has the advantage over other powder printing processes of a builtin metering system. That is, each cell of the gravure printing element provides the same amount of powder for each print made on the substrate from that cell, thus providing uniformly printed reproductions.

The process is conveniently carried out by depositing a suitable printing powder in the cells of a gravure print ing element, the powder being substantially free from electrostatic charges, applying an electrostatic charge of substantial magnitude to the powder in the cells, transferring the charged powder to a substrate in an electrostatic field of appropriate direction, and fixing the powder thus transferred to the substrate. The magnitude of the charges required and means for imparting them are well known to the art, as is the process of fixing resinous powders to substrates. The powders can be charged, for example, by induction if they are sufficiently conductive. On the other hand, if the powders are good insulators they can be charged 'by ion bombardment from a corona. Electrical fields can be established on conductive surfaces by direct application of potential, and on the surfaces of insulating materials by the deposition of charges.

In the apparatus for carrying out this invention it is preferable to reverse the doctor knife, that is, instead of the powder entering an actute angle formed between knife blade and cylinder surface, it enters an obtuse angle, the knife thus acting as a plow in scraping off excess.

Besides a conventional doctoring knife of hard or soft metal, rubber, plastic, etc., various types of cylindrical forms may be used. For example, a rotary rubber doctor, made by cutting grooves parallel to the axis of a rubber roller, was applied on the gravure cylinder at the doctoring station and rotated in a direction opposite to that of the cylinder rotation. The performance of this doctoring device was best when it was rotated at a surface speed several times that of the gravure cylinder. Another device functioning similarly is a rotary brush with bristles of great enough diameter to avoid penetration into the print cells. It was later found desirable to feed the powder to the gravure element by mounting a rotary brush in the powder reservoir discharge outlet.

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Such outlet should be located above the horizontal plane in which lies the axis of the cylinder bearing the gravure element, and should of course be adjacent to the element. It was also found desirable to loosen up any powder remaining in the printing cups after the image had been transferred to the substrate and before fresh powder was added from the reservoir. The brush in the reservoir performed its powder-feeding function and simultaneously the function of the powder-loosening brush. Means for this is preferably a rotary brush, the periphery of which is preferably moving in the same direction as the periphery of the cylinder bearing the gravure element and at a differential speed. Other means may be used for loosening the powder remaining after a print is made, such as nozzles or slots for blowing air into the printing cells.

In one embodiment of this invention there was used a sheet-fed press built around a print cylinder 5 /2 inches in diameter and 10 inches wide. The cylinder was driven by a power source operating through a variable-speed transmission of the planetary type rated at 0 to 400 r.p.m. Means for supplying powder to the gravure cells on the print cylinder were provided in a housing over the top half of the printing cylinder. Ions produced by corona from a single wire .004 inch in diameter located /2 inch from the printing cylinder charged the powder after it was doctored into the cells. .FOllOWing that station, a sheet of paper was guided into contact with the cylinder through a feed slot at the transfer station. Ions produced by corona from a pair of wires .004 inch in diameter, 1 inch apart and /2 inch from the printing cylinder, tacked the paper to the drum and effected transfer of the charged powder from the gravure cells to the paper, by virtue of the electrostatic field between the printing cylinder and the charged substrate. After it left the powder-transfer station, the paper was stripped from the cylinder by a pair of flat brass fingers or other suitable means. To prevent premature stripping of the tail of the sheet due to the weight of the part of the sheet already stripped, a curved plate was added to support the sheet and guide it to the sheet delivery tray. Also a small pinch roll was installed to help the paper match the speed of the print cylinder.

The patterns on the printing cylinder represented a variety of structures, including various sizes of dots and lines, a resolution chart, and halftone areas of -line, -line, and -line screens. Different areas of the patterns were etched to depths ranging up to 2 mils.

The corona discharge wire is supplied with a direct potential power supply having a voltage output variable to 8000 volts. Any suitable source of high direct voltage can be used. Besides a single charging wire or series of parallel wires there of course may be used a needle or series of needles, as is well known in the art. The preferred method of operation involves grounding the printing cylinder. The method may also be carried out when the printing cylinder is operated at a potential other than ground, provided the insulation is adequate and the necessary potential diiferences for producing corona from the charging wires are maintained.

It is desirable to loosen up any untransferred powder remaining in the printing cells between successive transfers. This may be efliciently done by adding a rotary brush to the equipment, the bristles having diameters sufiiciently narrow to enter the small printing cells as well as the large ones. Filament 3 mils in diameter, for instance, were satisfactory with 85-line to 150-line subjects, the cells having depths up to about 2 mils. The brush should have a peripheral. speed greater than the peripheral speed of the cylinderbrushed. When the peripheries of brush and cylinder travel in the same direction at the point of contact, although at different speeds, the loosening of powder is more efficiently done than with other arrangements. It is also possible to use an air doctor for loosening the powder. For example an air doctor with a slot aperture of 1 mil placed 5 mils from the roll was efficient. Too vigorous a blowing will not do because the vacuum pickup on the cleaning chamber may not handle the resulting volume of air and therefore dust would leak from the chamber. Such was the case when, for example, a slot with a 5-mil aperture was used 40 mils from the print cylinder. The slot of the air doctor should preferably be oriented for blowing normal to the surface of the cylinder.

Although the process is not limited to use of any particular powder material the preferred printing powders are combinations of resinous materials with colorants dispersed therein. Examples are Pliolite VTL contain ing 30% carbon black, Dow PS-Z" with 30% carbon black, a mixture of Dow PS-3" and Pliolite VTL in the proportions 3:2 together with Ultrasil and colorant.

The resins and colorant were ground in a Mikro- Atomizer." Fine-r grinds were obtained by regrinding the material in a fluid-energy Micronizer. Coarser grinds were obtained by screening a coarse precrush through ZOO-mesh and 325-mesh sieves (44 and 74 microns). There were thus obtained fractions with mean-mass sizes of 8 and 14 microns with Pliolite VT and mean-mass sizes of 4, 11, 18, and 42 microns with Dow PS-2. Pigment was milled into the resin before grinding.

Powders with mean-mass size of about 11 to 18 microns are preferred. Finer powder does not fill the printing cells as well and transfers less readily. Powders coarser than about 11 to 18 microns mean-mass size produce somewhat grainy prints.

Printing was done on paper, wood, cloth, and insulating substrates such as rubber, plastics, and ceramics.

Suitable resins for the process were numerous. Some performed better than others but it was found that the use of release agents improved the performance of many powders in cases where the transfers were otherwise not as clean as desired, Some of those resins found suitable were ethyl cellulose, Durez 19591, Piccolastic D-150, Pliolite S-SE, Pliolite VTL,'Saran F-120, Tylac CZ 210-4, Vinylite VYHH, Vinylite VYNS-B, Dow PS-Z (polystyrene), Epon 1004, Lucite 41, Saran F-220,'Vinylite VYLF, and Gelvatol 40-10.

A useful application of the process is in printing bottles or other glass forms while the glass is still hot from the forming molds. The temperature of the glass should be sufiicient to cause the powder applied to stick at the points of application. The optimum temperatures for adhesion will naturally vary with the softening range and the adhesive properties of the particular powder. Temperatures from about 300 to about 500 F. constitute a practical range for printing on glass with resinous printing powder. Printing powders with higher softening points and of suitable stability permit printing the hot glass at higher temperatures, the limit being where the glass is still too soft or the printing powder is too sensitive to the temperature. Bottles have been successfully printed at around 1200 F. by this method using frits as the powder, for example. One advantage of this embodiment of the invention is that the powder impinging on the hot surface of the glass sticks instantly, thus avoiding any undesirable shifting of the pattern. Thus, patterns applied to the hot glass are sharp, whereas if glass is used at a temperature below the sticking point of the powder, blurred images may be obtained due to bouncing of powders that charge inductively. Bouncing means the movement of powder several times back and forth between the gravure element and the substrate, thus degrading image definition in the print.

a Definitions By gravure engraving is meant a printing surface where the image is represented by depressions in the surface of the metal or other material forming the element, and the term includes intaglio, rotogravure, etchings, and the like.

Pliolite VTL is a vinyl-toluene/butadiene resin.

Dow PS-2 is polystyrene.

Ultrasil i very finely divided silica.

Piccolastic D-150 is a polystyrene resin.

Pliolite S-SE is a copolymer of 1 part by weight of butadiene and 6 parts by weight of styrene.

Saran F120 is a copolymer of vinylidene chloride and acrylonitrile.

Vinylite VYI-IH is a copolymer of 87% by weight of vinyl chloride and 13% by weight of vinyl acetate.

Vinylite VYNS-3 is a copolymer of 90% by weight of vinyl chloride and 10% by weight of vinyl acetate.

Epon 1004 is an epichlorohydrin/bisphenol A type solid epoxy resin having an epoxide equivalent of about 875-1025 and a melting point of about -105 C.

Lucite 41 is polymethylmethacrylate.

Saran F-200 is a vinylidene chloride/aerylonitrile copolymer.

Vinylite VYLF is a copolymer of 87% by weight of vinyl chloride and 13% by weight of vinyl acetate.

Gelvatol 40-10 is a polyvinyl alcohol with 37-42% of residual polyvinyl acetate.

Referring now to the drawings:

FIG. 1 represents schematically the printing mechanism of a sheet-fed press adapted for carrying out one embodiment of the process of this invention.

FIG. 2 represents sc-hematically another embodiment of the invention where 2 brushes are used.

FIG. 3 represents schematically an embodiment of the invention where one brush serves the double purpose of loosening the powder in the printing cells after each impression and supplying powder for filling them up to prepare for the next impression. Also shown is one embodiment of a mechanism for printing on hot glass bottles or the like.

In FIG. 1, 2 is an electrically grounded print cylinder having gravure engraving on its periphery, 3 represents a housing over the top part of the cylinder, 1 represents means for supplying powder to the gravure cells, with reverse doctoring, 4 is a wire for corona production, 5 is a sheet of paper for receiving the prints guided to the cylinder through 6, a feed solt, 7 is the transfer station, 8, 8 are corona wires for effecting transfer of the powder to the paper, 9 represents one or more flat fingers or other means for stripping the paper from the cylinder, 10 is the tail of the paper, 11 is a support and guide for the paper, 12 is a delivery tray to receive the printed paper, 13 is a pinch roll to assist in synchronizing the speed of the paper with that of the perip-hery of the cylinder, and 14 is a rotating brush for loosening the powder remaining in the printing cells after each print.

In FIG. 2, 1 is a doctor blade set up in reverse position, 2 is a powder reservoir box, 3 is a powder-feed rotary brush, 4 is an electrically grounded cylinder having gravure engraving on its periphery, 5 is a rotary brush for loosening powder remaining in the printing cells, 6, 6 are driving rollers, 7 is a high-voltage transfer roll, 8, 8 are feed guides, 9 is a substrate consisting of a board on which a wood grain pattern is to be printed, 10 is a precharge wire for imparting an electrostatic charge to the powder in the printing cells representing the pattern to be printed on the substrate, 11 indicates the transfer station, and 1.2 is the discharge outlet of the powder reservoir.

In FIG. 3, 1 is a powder-feed hopper, having 2 a rotating brush and 3 a doctor blade for sealing the feed hopper and removing excess powder from the non-printing areas of the cylinder, 4 is an electrically grounded printing cylinder having gravure engraving on its periphery, 5 is a corona unit for charging the powder with ions, the powder then being transferred to 6, a hot glass bottle, as the powder is brought into close proximity with the hot glass surface at the transfer point where an electrical field of sufficient strength and direction is provided to bring about the transfer, 7, 7 are drive rolls of electrically and thermally insulating material for moving the periphery of the bottle in synchronism with the periphery of the print cylinder, 8, 8 are bed rolls for supporting the bottle and applying high voltage to it to establish the electrical field necessary to effect printing.

What is claimed is:

1. The method of gravure printing wherein an image is formed on a substrate with a dry printing powder which comprises:

(a) furnishing to cells etched in the surface of a rotating gravure cylinder a printing powder adapted to receive and retain an applied electrostatic charge;

(b) creating an electrostatic field at a point traversed by the surface of said gravure cylinder after the printing powder is furnished to the cells etched therein;

(c) feeding a substrate so that it travels through said field at substantially the same rate and direction as the etched surface of said gravure cylinder and in proximity to said etched surface;

(d) applying an electrostatic charge to the powder in said cells at a point located between the point where the powder is furnished to said cells and the point where said cells travel through said electrostatic field;

(e) rotating said cylinder at such speed that a substantial portion of the electrostatic charge applied to said powder is retained on said powder as it enters said electrostatic field, so that said printing powder is transferred from said cells to said substrate under the influence of said electrostatic field; and

(f) then fixing the powder on the surface of said substrate.

2. The method of gravure printing described in claim 1, with the further limitation that, immediately after the powder is deposited on the cylinder, the excess powder is removed from the cylinder by reverse doctoring.

3. The method of gravure printing described in claim 1, with the further limitation that, after the transfer of powder to the substrate and before the deposition of more powder into the cells from which powder has been transferred to the substrate, the powder remaining in the cells is loosened.

4. The method of gravure printing described in claim 3, with the further limitation that, immediately after the powder is deposited on the cylinder, the excess powder is removed from the cylinder by reverse doctoring.

5. A printing apparatus for gravure printing wherein an image is formed on a substrate with a drying printing powder which comprises:

(a) a rotatable printing cylinder having thereon a gravure-type engraving,

(b) a powder reservoir having a discharge outlet located adjacent to the cylinder and above the horizontal plane wherein the axis of said cylinder lies,

(c) d-octoring means in contact with the surface of the gravure-type engraving at a station located after the reservoir discharge outlet in the sense of the direction of travel of the cylinder surface,

(d) electrostatic charging means located at a station after the doctoring means in the sense of the direction of travel of the cylinder surface, N

(e) means for bringing a substrate into close proximity with the charged powder, the point of closest proximity being located beyond the powder-charging station in the sense of the direction of travel of the cylinder surface,

(f) means for establishing an electrostatic field between the charged powder and the substrate when the latter is brought into close proximity with the powder, and

(g) a rotary brush means for loosening the powder remaining in the gravure cells after the cells have passed through the electrostatic field,

(h) means for fixing the powder image on the substrate after the transfer has been made.

6. A printing apparatus for gravure printing as in claim 5 wherein the said rotary brush means for loosening the powder is located in the powder reservoir.

7. A printing apparatus for gravure printing as in claim 5 wherein the said rotary brush means for loosening the powder is located between the transfer station and the reservoir station.

8. A printing apparatus for gravure printing as in claim 5 wherein the said electrostatic charging means is a corona discharge device.

9. The method of printing wherein an image is formed on a substrate with a dry printing powder which comprises:

(a) supplying a printing powder, adapted to receive and retain an electrostatic charge, to the upper surface of a gravure printing plate so that it fills the cells in said plate; thereafter (b) doctoring said surface of the printing plate to wipe excess powder therefrom; then (0) depositing an electrostatic charge on the surface of the powder in said cells;

((1) bringing a surface of the substrate into proximity with the powder while said powder still retains charge; and

(e) creating, at the location where said substrate is in proximity with the charged powder and while said powder still retains charge, an electrostatic field of such strength that said printing powder is transferred from said cells to said substrate under the influence of said electrostatic field.

References Cited by the Examiner UNITED STATES PATENTS 2,067,949 1/ 1937 Rez.

2,148,456 2/1939 Grossarth 101-169 2,162,317 6/1939 Rez.

2,318,504 5/1943 Lodding 101-169 X 2,408,143 9/1946 Huebner.

2,576,047 11/ 1951 Schaffert.

2,752,271 6/ 1956 Walkup et a1.

2,787,556 4/1957 Haas.

2,987,037 6/ 1961 Bolton 118637 OTHER REFERENCES Decoration of Glass and Ceramic Articles by Xerography by C. D. Oughton, article in The Glass In= dustry, December 1949, pages 662-664 and 684.

ROBERT E. PULFREY, Primary Examiner,

S: URR; A am" Exam n 

1. THE METHOD OF GRAVURE PRINTING WHEREIN AN IMAGE IS FORMED ON A SUBSTRATE WITH A DRY PRINTING POWDER WHICH COMPRISES: (A) FURNISHING TO CELLS ETCHED IN THE SURFACE OF A ROTATING GRAVURE CYLINDER A PRINTING POWDER ADAPTED TO RECEIVE AND RETAIN AN APPLIED ELECTROSTATIC CHARGE; (B) CREATING AN ELECTROSTATIC FIELD AT A POINT TRAVERSED BY THE SURFACE OF SAID GRAVURE CYLINDER AFTER THE PRINTING POWDER IS FURNISHED TO THE CELLS ETCHED THEREIN; (C) FEEDING A SUBSTRATE SO THAT IT TRAVELS THROUGH SAID FIELD AT SUBSTANTIALLY THE SAME RATE AND DIRECTION AS THE ETCHED SURFACE OF SAID GRAVURE CYLINDER AND IN PROXIMITY TO SAID ETCHED SURFACE; (D) APPLYING AN ELECTROSTATIC CHARGE TO THE POWDER IN SAID CELLS AT A POINT LOCATED BETWEEN THE POINT WHERE THE POWDER IS FURNISHED TO SAID CELLS AND THE POINT WHERE SAID CELLS TRAVEL THROUGH SAID ELECTROSTATIC FIELD; (E) ROTATING SAID CYLINDER AT SUCH SPEED THAT A SUBSTANTIAL PORTION OF THE ELECTROSTATIC CHARGE APPLIED TO SAID POWDER IS RETAINED ON SAID POWDER AS IT ENTERS SAID ELECTROSTATIC FIELD, SO THAT SAID PRINTING POWDER IS TRANSFERRED FROM SAID CELLS TO SAID SUBSTRATE UNDER THE INFLUENCE OF SAID ELECTRASTIC FIELD; AND (F) THEN FIXING THE POWDER ON THE SURFACE OF SAID SUBSTRATE. 